1. Field of the Invention
The invention relates to a process for the preparation of 3-substituted thiophenes in which the substituent consists of a C.sub.4 -C.sub.18 alkyl group.
2. Description of the Related Art
It is known that the preparation of 3-substituted thiophenes is more difficult than the preparation of 2-substituted thiophenes. However, 3-substituted thiophenes are a desirable intermediate or final product in various applications, for example as an herbicide or a fungicide. Certain 3-substituted thiophenes can also be used as monomers in the preparation of, for example, electrically conductive polymers.
3-substituted thiophenes can, in principle, be prepared in two different ways: by using as the initial material a thiophene or a thiophene provided with a modifiable 3-substituent, or by using as the initial material an acyclic compound. Substituted thiophenes can, of course, also be prepared by other methods, but in that case the products are usually 2-substituted.
The best known way of using thiophene as the initial material is the bromination of thiophene. 3-Bromothiophene can be prepared, for example according to EP publication 299 586, catalytically from 2,5-dibromothiophene and thiophene. 3-Bromothiophene is one of the most important intermediates from which a wide variety of 3-substituted thiophenes are obtained, for example by means of Grignard reactions, by lithiation, or by oxidation. The use of 3-bromothiophene as the initial material in a production process is, however, hampered by the undesirable bromides formed as byproducts and by the difficult reaction conditions, such as an unsuitable temperature and an inert atmosphere.
The preparation of 3-substituted thiophenes by using an acyclic compound as the initial material has been studied extensively. These processes often have the disadvantage that they produce large amounts of 2-substituted thiophenes as by products. Some examples of these processes are the use of various mercaptans or thioethers as the initial material (Butler, A. R., Henry, J. B., J. Chem. Soc., B, 1970, or Campaigne, E., Monroe, P. A., J. Am. Chem. Soc., 76 (1954), 2445). 3-substituted thiophenes can also be prepared from other compounds which contain either double bonds or other functional groups (Veal, K. T., Grinter, T. J., EP 38121, or Duus, F., Tetrahedron, 32 (1976), 2817).
As mentioned above, many of these processes are known to those skilled in the art to produce large amounts of undesirable 2-substituted thiophene compounds and isomers, the separating of which causes difficulties. Furthermore, the undesirable byproducts are derived from the same initial material, and thus the yield will remain very low in the state-of-the-art processes.